1. Field of the Invention
The present invention relates to a heat-resistant magnesium alloy which can inhibit cracks when being cast, and which is good in terms of the castability and heat resistance.
2. Description of the Related Art
Due to the recent increasing weight reduction need, magnesium alloys which are much lighter than aluminum alloys have been attracting the attention of engineers. Magnesium alloys are the most lightweight among practical metals, and are about to be used as automotive materials in addition to aeronautical materials. For example, magnesium alloys are used in automobile wheels and engine head covers. In addition to these, as the environmental consciousness has been enhanced recently, it has been required to further make vehicles lightweight. Accordingly, it has come to examine to use magnesium alloys even for appliances or apparatuses which are used in high-temperature regions. In this instance, the heat resistance of magnesium alloys matters naturally. For instance, AZ91 as per Japanese Industrial Standard, a general magnesium alloy, exhibits such a very low creep strength that it is not appropriate for component parts which are used in high temperature environments. Consequently, AE42 as per the standard of the Dow Chemical Company of the United States of America is available as one of such materials whose heat resistance is improved. Moreover, Japanese Patent Publication No. 3,229,954, Japanese Unexamined Patent Publication (KOKAI) No. 2002-129,272 and Japanese Unexamined Patent Publication (KOKAI) No. 2002-275,569 propose magnesium alloys which are good in terms of the anti-creep strength or the other properties.
All of these magnesium alloys include rare-earth elements (hereinafter abbreviated to as “R.E.” wherever appropriate) in an amount of from 0.5 to 3% by mass approximately. Rare-earth elements are surely effective elements in improving the heat resistance of magnesium alloys.
However, rare-earth elements are so expensive that they raise the cost of magnesium alloys and their cast products. Moreover, according to the survey and study carried out by the present inventors, rare-earth elements are found to be elements which are likely to cause cracks in casting alloys. Accordingly, it is not preferable to have rare-earth elements contained in cast alloys. In addition, the present inventors have come to know newly that it is possible to produce heat resistance, such as sufficient creep resistance, without including rare-earth elements in magnesium alloys.